In cosmology, the
word denotes ordinary matter in contrast with more exotic forms of matter
that the greatest part of dark matter
is thought to consist of. This usage comes about because cosmologists
are mainly interested in what percentage of mass in the universe is
represented by ordinary matter. The mass of ordinary matter is
mostly contained in atomic nuclei, and these nuclei are built of baryons (protons and neutrons),
the total baryonic mass is, to good approximation, the same as the
total mass of ordinary matter in any region of our universe.

big bang

The big bang models are the foundation of modern cosmology. Firmly grounded in Einstein's theory of general relativity, they describe a universe that began in a very hot initial state and has expanded (and cooled down) ever since. They make precise predictions about nucleosynthesis in the early universe, the existence and properties of the cosmic background radiation, and the distribution of distant galaxies in the cosmos, which have been confirmed by astronomical observation.

The word "big bang" has two different meanings. In a strict sense, the big bang is a space-time singularity, a state of infinite density - the initial state the big bang models predict for our universe. In a more general sense, the term is applied to the earliest cosmic eras, in which the universe was exceedingly hot and dense. Further information about these two meanings and why it is important to distinguish between them can be found in the spotlight text A tale of two big bangs.

Synonym: primordial nucleosynthesis. The formation of complicated nuclei from constitutents such as protons and neutrons in the early universe. According to the big bang models, the early universe was filled with a particle soup of protons and neutrons. At cosmic times between a few seconds and a few minutes, nuclear reactions produced the first light elements, mainly nuclei of deuterium, different varieties of helium and lithium.

A system consisting of two stars in orbit around each other.
From a relativistic point of view, there are binaries that are of
special interest, namely those in which at least one of the partners
is a neutron star
or a black hole.
Potentially, such systems are effective sources of
gravitational
waves.

binding energy

The energy needed to break up a composite object into its component parts.

A theorem of general relativity,
discovered by J. T. Jebsen (1921) and independently discovered, and
named after, George D. Birkhoff (1923): Any spherically symmetric
spacetime has the same properties as some region from one of a simple
family of spacetimes found by Karl Schwarzschild in 1916. More
concretely: The spherically symmetric spacetime around any spherically
symmetric matter configuration has the same properties as space-time
around a Schwarzschild black hole of the appropriate mass.

BKL conjecture

Conjecture by the Soviet physicists Vladimir Belinskii, Isaak Khalatnikov and Evgeny Lifshitz that, near a singularity, the contribution of matter to gravity becomes negligible compared with the effects of gravity as a source of further gravity (compare the spotlight text The gravity of gravity), and that near a singularity, the variation of the gravitational field from one location to the next can be neglected - what is much more important is the way gravity changes over time. Further information about this can be found in the spotlight text Of singularities and breadmaking.

black hole

Region of space in which a sufficient amount of mass is concentrated to form a gravitational prison - a region into which matter or light can enter from the outside, but from which nothing that has ever fallen in can ever leave.

In Einstein's theory, black holes are truly black, due to the fact that no radiation or light can ever escape them. Once quantum theory is taken into account, that need no longer be true - on the contrary, it seems as if black holes should emit so-called Hawking radiation. However, for astrophysical black holes (that typically have more or even much more than one solar mass), that radiation would be undetectable if we could transport today's finest sensors into the immediate vicinity of the black hole.

black hole uniqueness theorems

Theorems proved in the context of general relativity that answer the question: How many different kinds of black holes are there? If that question is restricted to stationary black holes (namely black holes that have settled down and do not change over time), then the answer is: Surprisingly few. Once you know a stationary black hole's mass, angular momentum (roughly speaking, how fast it rotates) and electric charge, its properties are determined completely.

The frequency of a simple light wave is directly related to its colour (cf.
spectrum). For the highest
possible frequencies, the colour is blue-violet. If the frequency of a light
wave is shifted towards higher frequencies (for instance by the
doppler shift), that
corresponds to a colour shift towards the blue-violet end
of the spectrum, and is hence called a blue-shift.

From this, "blue-shift" has come to acquire a more general meaning.
It is used to denote any shift towards higher frequencies, even
for types of electromagnetic radiation where the frequencies do not
correspond to any visible colour, and more generally still, for
other types of waves as well (for instance for
gravitational waves).

In string theory:
An object that is the analogue of a two-dimensional membrane embedded
in three-dimensional space - an entity with a certain number of dimensions
(one-brane, two-brane, three-brane...) embedded in the
higher-dimensional space of string theory. A one-brane or 1-brane has
one spatial dimension, a two-brane has two, and so on.

brane world

The notion that our world with its three dimensions of space is a three-brane embedded in a higher-dimensional space, akin to a two-dimensional surface embedded in ordinary three-dimensional space.

A failed star: A gas ball
in space that has between one and ten percent solar mass - not enough for the temperature and pressure
in its core to reach the values required for the nuclear fusion to start that would transform the gas ball into a shining star.